The present invention relates to the use of a combination of laser energy and ultrasonic energy for soldering an insulation coated wire to a solder covered pad located on a heat-sensitive substrate, and particularly to the stripping and soldering of fine insulated wires for connecting a magnetic head to a circuit carrier substrate.
Packaging of magnetic heads requires very delicate wire connecting operations in order to interconnect the magnetic head component to a circuit carrier for providing electrical signals to and receiving electrical signals from the magnetic head.
Presently, insulation is removed from insulated wire by conventional wire stripping methods, such as plasma or laser stripping. The bare wire is subsequently soldered or otherwise jointed to a metallic pad located on the magnetic head. While this process is generally successful, future developments and product demands will require improved manufacturing methods. For example, the constant industry migration toward higher storage densities requires the use of very fine wires and smaller pad dimensions. All the while, there is constant demand for improving manufacturability and reducing costs.
It is doubtful that existing ultrasonic bonding and thermode-reflow techniques currently in use will meet future joining requirements. While wire-soldering itself is a well established technology, conventional methods require the steps of removing any insulation from the wire, applying flux to the bare wire and then applying high intensity heat pulses for a relatively long-time duration. The combination of steps described are detrimental to delicate electrical assemblies which are sensitive to heat and corrosion damage. Furthermore, experience with electrical thermodes indicates that some heat is applied indirectly to the wire through melting of the solder and there is occasional solder splattering and solder bridging short circuits. Another problem with conventional solder heating is solder embrittlement at the bonding interface due to the relatively long heat exposure time of several seconds during soldering.
The prior art contains proposed solutions to these problems. For example, U.S. Pat. No. 4,893,742 issued to P. Bullock describes fluxless soldering of a wire to a pad by the combined application of ultrasonic energy and laser energy. The patent describes the joining of a bare lead to a pad.
U.S. Pat. No. 4,818,322 issued to R. Morino et al describes the use of laser energy for bonding and stripping conductors during a scribing operation. Laser energy is used to achieve insulation stripping, soldering and cutting of conductors at terminal points. Ultrasonic energy is supplied to provide a relatively small frictionless wire guide and to press the wire into an activated adhesive. The ultrasonic energy does not contribute significantly to the energy supplied for activating the adhesive.
U.S. Pat. No. 3,789,183 issued to L. Conley describes the application of ultrasonic vibrations to scrub away the insulation from a wire prior to applying a welding current to weld the wire to a terminal.
The present invention provides a solution to the problem of fluxless soldering of insulated wires to pads without heat damage exposure of delicate electrical devices located near the bonding pad and to the substrate supporting the pad. A bonding or soldering tip is ultrasonically vibrated while the tip is being heated with laser energy to remove the insulation from a wire and then melt solder in order to join the wire to a pad located on a substrate without the addition of any flux. The laser energy is highly localized and of a sufficiently low-energy to avoid heat damage to either electrical devices or the substrate.